1. Field of the Invention
This invention relates to a general scanning-period converting device for converting a scanning period (frequency) of a video signal to another scanning period.
2. Description of Related Art
Conventionally, conversion processing of an original video signal to a video signal having a scanning frequency which is integer times as high as the scanning frequency of the original video signal, such as mode conversion processing between different television systems and image signal conversion processing for multi-image display, etc., has been easily performed even between different types of video signals. Such scanning-period (frequency) conversion processing has been also recently required to convert an original video signal to a video signal having a scanning period which is not necessarily integer times of that of the original video signal, for example in a case where an output character video signal of a computer is composed with a video signal and then the composite video signal is displayed on an exclusive display for the computer. The following two cases are considered as example of the above case. In one case, a video signal of a video source is composed with an output of each of various different kinds of computers, and each composite signal is displayed on an exclusive display for the corresponding computer. In the other case, a video signal of a video source is composed with an output of any one of various different kinds of computers, and the composite signal is selectively displayed on a multi-scan type of display.
In the above cases, in order to treat the original video signal at the same frequency as a currently-connected computer at all times, a different type of scanning-period converter is required in accordance with variation of the video scanning-period of the computer (that is, in accordance with a switching operation for connection with each computer). This requirement is inconvenient and causes the device cost to be high.